Any conditioned response is the expression of a specialized learning system with evolutionary and neurological significance. Gustatory cues are effective signals (CS) when paired with internal visceral consequences (US), but ineffective when paired with peripheral pain (US). On the other hand, telereceptive (visual-auditory) cues (CS) are effective for peripheral pain, but not for changes in internal states. Brain stem emetic areas mediate the acquisition and extinction of conditioned taste aversions. Subdiaphragmatic vagotomy blocks the majority of neural input from the gut to the brain. It also blocks the acquisition of flavor aversions when the toxin is a gastric irritant, but not when the toxin is a centrally-active agent like apomorphine or alcohol. Vagotomy also facilitates extinction of conditioned flavor aversions, but not conditioned fear in buzzer shock conditioning, probably by blocking conditioned emetic reactions. We are using pharmacological and nerve lesion techniques to separate the anatomical substrates of flavor-illness from that of buzzer-shock conditioning. We are also determining the behavioral roles of olfaction of rats. Olfaction serves a dual function, being involved in both food selection and peripheral defense. However, if odor is conditioned in compound with taste, odor now becomes as strong as taste. Taste potentiates odor. Association of odor and taste at the time of poisoning is critical to potentiation: We are using behavioral preparations to test the hypothesis that taste indexes odor as a feeding cue, which mediates an illness-induced odor aversion.